Comments section overruns on top of sidebar

For the first time I have more than 20 comments against one of my posts, but now the comments appear to be spilling over on to the right hand side of the page and appearing where the sidebar should be. The sidebar is right down the bottom of the page below everything else:

I've played with the comments settings to try and show less than 20 comments per page but ideally I'd just have all comments continuing below the main article and the sidebar back in its rightful place.

Theme is Enterprise from StudioPress. Anyone got any ideas? I appreciate it might just be me being incredible stupid.